French President Francois Hollande said Wednesday that growing evidence of the use of chemical weapons in Syria "obliges the international community to act."
The French leader's comments came as the US and Russia again failed to come to a consensus on the Syrian conflict, which has killed an estimated 80,000 people over the past two years.
International efforts to resolve the conflict between Syrian President Bashar al-Assad, whose allies include Russia and Iran, and an armed rebellion against his rule have failed.
Lakhdar Brahimi, the United Nations' and Arab League envoy to Syria, described the lack of international will on Syria as "embarrassing," while Hollande tempered his call by saying any response must be "within the framework of international law."
Assad's forces, meanwhile, have recaptured the strategic town of Qusair following weeks of brutal assault on the formerly rebel-held city.
The White House on Wednesday condemned the move in no uncertain terms. Assad's offensive there "killed untold numbers of civilians and is causing tremendous humanitarian suffering," White House spokesman Jay Carney said in a statement. "It is clear that the regime could not contest the opposition's control of Qusair on its own, and is depending upon Hezbollah and Iran to do its work for it in Qusair," he added.
The comments from the US came a day after French Foreign Minister Laurent Fabius claimed to have conclusive evidence sarin gas was used in Syria. Laboratory tests completed in France "prove the presence of sarin in the samples in our possession," he said in a statement Tuesday, saying it "now is certain that sarin gas was used in Syria multiple times and in a localized way."
The statement did not get into whom is believed responsible for using the deadly nerve agent in Syria, but did say "[i]t would be unacceptable that those guilty of these crimes benefit from impunity." Also Tuesday, Britain said it had also found evidence of sarin gas use in Syria.
Assad and the rebels have traded accusations over the use of chemical weapons in Syria, prompting an inquiry from the UN.
Hollande's comments follow a UN report on Syria released Tuesday, which found limited quantities of unknown chemical agents had been used in at least four attacks there. The UN appraisal also found proof of war crimes and crimes against humanity.
The Commission of Inquiry on Syria, tasked with investigating human rights violations, said it had "reasonable grounds" to believe rebels have used toxic agents, although it said the "majority" of attacks were probably committed by Assad's forces.
The UN report said the four chemical attacks are believed to have hit two Khan al-Asal neighborhoods, Aleppo and Uteibah, on March 19, the Sheikh Maqsood district of Aleppo on April 13, and the northwestern town of Saraqab on April 29.
Syria's UN ambassador Faysal Khabbaz Hamoui dismissed the report on Tuesday, saying the commission "excessively exaggerated their conclusions and outcomes" and "totally neglects the substantial events, or even marginalizes them."
The UN commission called on the international community to find a diplomatic solution that would include all Syrians, insisting that world leaders restrict arms sales there.
The European Union recently lifted an arms embargo on Syria, while Russia reportedly sold Assad S-300 surface-to-air missile defense systems, though is not clear when they are set to arrive in Damascus.